How Long Does it Take for a Blood Culture to Turn Positive in a Septic or Bacteremic Infant? Study Answers the Question

Since the age of the dinosaurs (ok the last 30 years) we have been observing sick infants (typically those under 2 months who have a fever) in the hospital after drawing blood cultures, cerebral spinal fluid, urine, getting a chest X-ray (CXR) and putting them on antibiotics for a minimum of 48 hours. This is what in doctor speak was a “septic work-up”. Think of it as insurance because if we didn’t do this, it could be your child that actually has the potentially fatal infection. What we know is that a percentage of these infants would have died of overwhelming infection if left untreated. That percentage is relatively low. Depending on how sick they look and other variables, it could be as low as 2% or in the very ill infant, obviously much higher.

The standard has always been that you keep these infants on IV antibiotics for at least 48 hours while you wait for the blood culture results. The study, “Blood culture time to positivity in febrile infants with bacteremia,” (which you can read here… ) looked at almost 400 positive blood cultures in infants under 90 days of age and found that 91% were positive by 24 hours, 96% by 36 hours and 99% by 48 hours. The authors conclude “inpatient observation of febrile infants for more than 24 hours may be unnecessary in most infants.”

Since these are the sickest of sick infants, most at risk of overwhelming infection, I would make a different conclusion, which is that the 48 hour observation time on antibiotics is perfect as it will pick up 99% of those infants who needed to be on antibiotics. Sending infants home after 24 hours would miss 9% of the sickest infants, potentially sending them home to end up with overwhelming infection. Now the truth is that the 24 hours of antibiotics may have actually been enough to kill the bacteria and those 9% might end up fine anyway. That is the topic of a yet to be done study, that we likely can never do as it would not be ethical to put a group of infected infants on the untreated arm of the study just to see if they were cured by one day of antibiotics.

So for now, I recommend we stay with the 48 hours sepsis work-up. There is a simple and likely very effective compromise. We have a very effective antibiotic that has the ability to be dosed once a day due to its very long half-life; ceftriaxone. Infants with negative blood cultures at 24 hours, could be given a dose of ceftriaxone and then seen in a day. This won’t cover the very rare cases of sepsis and bacteremia from the organism listeria, but would cover most other causes of infection in this age group.

Consult with your physician of course. This blog is not meant to change standard of care, merely passing on interesting information.